1. Field of the Invention
The present invention relates to a system and method for automatically controlling vehicle cruising speed, particularly the system and method for automatically controlling vehicle speed in such a way that either the vehicle moves constantly at a predetermined vehicle speed when there are no other vehicles in front of the given vehicle in which the system is mounted or the given vehicle automatically tracks the other vehicle immediately in front thereof.
2. Description of the Prior Art
Recently, there are increased numbers of automotive vehicles which employ a system for holding the vehicle at a constant predetermined speed. Consequently, in order to add to the versatility of such systems, it has been proposed that such systems control the vehicle running speed so that the vehicle follows other vehicles in front of it according to the detected distance to the preceding vehicle, thus preventing collisions with the other vehicle and generally improving drivability.
Such a system is disclosed in Japanese Patent Application Publication No. 55-86000, the disclosure of which is hereby incorporated by reference.
In more detail, the proposed system is so designed that the vehicle is forced to move at a set speed when the distance between the vehicle and the vehicle immediately in front of it exceeds a safety margin determined according to the set vehicle speed and that, the vehicle speed is adjusted so that the vehicle tracks the other vehicle at the same vehicle speed when the inter-vehicle distance drops below the safety margin.
In conventional vehicle cruise control systems, the system is disabled when the vehicle speed deviates from the set speed by more than a predetermined speed in order to prevent overload of the engine when no other vehicle is present in front thereof, i.e., out of the safety margin. However, such a disabling function incorporated in the system is not performed in a case when the front vehicle is moving at a low speed even if the actual vehicle speed is below a lowest limit with respect to the set vehicle speed while the vehicle tracks the front vehicle.
In addition, there is a problem that in a case when the front vehicle cruises below the lowest limit speed and has suddenly disappeared out of a range in which the distance from a front vehicle can be measured, e.g., at an intersection during the automatic tracking for the front vehicle, the system is disabled by the above-described disabling function since the vehicle speed is below the lowest limit with respect to the set speed and consequently the vehicle driver must purposely operate the vehicle and system thereof to reach the set vehicle speed.